1. Technical Field
This invention pertains to seals for turbine engines in general and to brush seals in particular.
2. Background Information
Brush seals are designed to provide a controlled leakage flow from a higher pressure region to a lower pressure region through a gap disposed between a stationary member and a rotating member. The sealing is providing by a plurality of bristles packed between a side plate and a back plate. The side plate is located on the high pressure (PH) side of the seal and the back plate is located on the low pressure (PL) side of the seal. The bristles extend beyond the side and back plates, across the gap between the stationary and rotating members.
Referring to FIG. 1, one form of brush seal deterioration that can result in higher than expected levels of seal leakage is described below. Generally the leakage flow velocity component in the aft direction through a brush seal bristle pack 104 is nearly independent of radial location (assuming a uniform profile) at each of the upstream 102 and downstream 105 surfaces of the bristle pack 104. Due to seal stiffness and pressure requirements, in a traditional brush seal 100 the area of bristle pack upstream surface 102 exposed to the higher pressure PH is typically much larger than the area of bristle pack downstream surface 105 exposed to the lower pressure PL. From flow continuity it can be shown that the average aft component of fluid velocity is significantly larger at the downstream surface 105 than at the upstream surface 102. Because the forces acting on the bristles in the flow direction are expected to increase with increasing flow velocity, it is recognizable that the aft most bristles experience the highest forces. Such loading causes high bending stress in the bristles at the cantilever formed by the backing plate, and may eventually lead to bristle failure that progresses from aft forward. A person of skill in the art will recognize that an increased leakage rate though brush seals within a gas turbine engine can directly decrease the thrust specific fuel consumption (TSFC) of the engine and consequently the efficiency of the engine.
What is needed is a durable brush seal that can be used in a gas turbine engine environment, and one that maintains its designed leakage flow rate during operation.
According to the present invention, a brush seal is provided having a side plate, a back plate, and a bristle pack disposed between the side plate and the back plate. The side plate is in contact with an upstream surface of the bristle pack. The back plate is in contact with a downstream surface of the bristle pack. The back plate includes a plurality of apertures positioned to provide a gas path through the back plate for gas exiting the bristle pack. In some embodiments, the back plate further includes an exit channel disposed in the back plate surface contiguous with the bristle pack, aligned with the apertures. In some embodiments, the radial length of the side plate in contact with the bristle pack is less than the radial length of the back plate in contact with the bristle pack.
An advantage of the present invention is that an increased durability brush seal is provided. Under normal operating conditions, traditional brush seals can prematurely deform by deflecting in the direction of the leakage flow through the seal. This type of deformation is a function of the pressure difference across the seal and more specifically the velocity of the flow passing through the bristle pack. In a traditional brush seal having a side plate radially shorter than the back plate, the velocity of the leakage flow in the region adjacent the edge of the back plate is appreciably higher than elsewhere in the bristle pack. As a result, the bristles in the region adjacent the edge of back plate are subject to the highest loading, which increases their susceptibility to deformation. In the event of such a deformation, the leakage flow rate is dramatically increased and the seal function comprised. The present brush seal substantially decreases velocity of the leakage flow in the region adjacent the edge of the back plate by providing an alternative leakage path through the back plate of the brush seal. As a result, the bristle loading in the region adjacent the edge of back plate is less than in prior art brush seals of which we are aware. In addition, leakage flow through the bristle pack region between the back plate and the sealing surface is also less than in prior art brush seals, thereby further reducing the loading on the bristle pack and the consequent probability of deformation.
These and other objects, features and advantages of the present invention will become apparent in light of the detailed description of the best mode embodiment thereof, as illustrated in the accompanying drawings.